The present invention relates to variable displacement hydrostatic pumps, and more particularly, to such pumps of the type wherein the pump displacement is controlled, either directly or indirectly, in response to movement of a manual input member (control lever).
Many variable displacement hydrostatic pumps of the type to which the present invention relates are utilized in the field of mobile hydraulics, i.e., as part of the hydraulic system of various types of vehicles. Furthermore, many such pumps are utilized in the propel circuit of the vehicle, to supply fluid under pressure to a fluid motor which transmits torque to the drive wheels of the vehicle. Therefore, although the present invention is not limited to use in a pump which is part of a vehicle propel circuit, such is an especially advantageous application of the invention, and the invention will be described in connection with such a circuit.
It is typical for vehicles having hydrostatic propel circuits also to include one or more devices or functions which require some assurance that the hydrostatic pump is in neutral. For example, if the vehicle is equipped with brakes having hill-holding capability, it is important that the pump not be displaced from its neutral position whenever the brakes have been applied. In other words, it would not be desirable to permit the vehicle to "drive through" the brakes.
As another example, many such vehicles are equipped with an operator seat switch which serves as a safety feature by sensing the presence of the vehicle operator in the seat and sending an electrical signal to enable operation of various vehicle functions, but only in response to the operator being in the seat. For example, a number of vehicles have been provided with an electrical operator seat switch, connected to the engine ignition system, such that the vehicle engine can not be started unless the operator is sitting in the operator seat. Such an arrangement is helpful, but once the engine is already running, the seat switch has effectively been bypassed, and the pump can be displaced in situations in which displacement of the pump is not appropriate.
On vehicles having variable displacement hydrostatic pumps as part of the propel circuit, wherein pump displacement is varied by manual movement of a manual control lever, one of the problems in the operation of the vehicle is that, periodically, the operator or someone else will accidentally move the manual control lever in a situation in which movement of the vehicle is not desired. For example, as the vehicle operator is climbing onto the vehicle, in preparation for sitting in the operator seat, if the operator accidentally grabs or bumps the control lever, the vehicle will begin to move, and perhaps rather suddenly, in either a forward or reverse direction. With the operator not expecting the vehicle to move at that time, and perhaps not being in the operator seat, such premature movement of the vehicle can present a safety concern.